As is known in the art, as turbocharged, downsized engines become a major strategy to improve vehicle fuel economy; improvement of their transient responses is very important. When a turbocharger is sized relatively small for rapid transient response, it may run out of flow capacity at high power thus limiting its rated power; on the other hand, when a turbocharger is sized large enough to deliver competitive power, it may perform very sluggishly during transients. There are several technologies that have been suggested to solve this dichotomy, e.g. use of a serial sequential turbocharger system, mechanical driven superchargers, or electrical driven superchargers or turbochargers.
As is also known, in conventional turbocharged engines, engine and turbocharger work independently with each other. The “lack of synchronization” pushes the compressor into a less efficient operating region during transient operation and drives the turbine to work with low speed ratios (U/C) (where U/C is the blade-speed ratio) thus operate in a lower efficiency operating region (i.e. the hydraulically governed turbocharger will allow the compressor and turbine to be designed for higher efficiency since they don't have to trade the efficiency for operation range).